Flash memory devices represent one type of nonvolatile memory that typically utilize arrays of EEPROM cells. In particular, a flash memory is a form of EEPROM technology that allows multiple memory locations to be erased or programmed in one programming operation. Normal EEPROM typically only allows one location at a time to be erased or programmed, which means that flash memories can operate at higher effective speeds when the systems using them read and write to different locations at the same time. Flash memory is typically made in two forms: NOR-based flash and NAND-based flash. The names NOR and NAND refer to the type of logic gate used in each storage cell. NOR-based flash was the first type to be developed. It has relatively long erase and write times, but has a full address/data interface that allows random access to any location. This makes it suitable for storage of program code, which only needs to be infrequently updated. NOR-based flash may have an endurance in a range between 10,000 and 100,000 erase cycles. NOR-based flash was the basis for early flash-based removable media, including both Compact Flash and Smart Media. In contrast, NAND-based flash typically has faster erase and write (i.e., program) times, higher density and lower cost per bit than NOR-based flash and much greater endurance. However, its I/O interface typically only allows for sequential data access. This makes it suitable for mass-storage devices such as PC cards. NAND-based flash has led to several small media formats, including MMC, Secure Digital and Memory Stick. NAND-based flash forms the core of the removable USB interface storage known as key drives.
Flash and other nonvolatile memory devices have also been developed with write protection features that reduce the likelihood of mistakenly erasing or overwriting data to be protected. Examples of nonvolatile memory devices having write protection are disclosed in U.S. Pat. No. 6,031,757 to Chuang et al., U.S. Pat. No. 5,513,136 to Fandrich et al. and U.S. Pat. No. 5,197,034 to Fandrich et al. Unfortunately, conventional write protection techniques typically require locking one or more fixed sized blocks within a memory array and/or the use of additional external pins to control the locking operations. Thus, notwithstanding these conventional nonvolatile memory devices, there continues to be a need for improved nonvolatile memory devices having flexible write protection characteristics that do require the use of additional pins or limit protection to fixed sized blocks.